Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

1 files changed, 117 insertions, 0 deletions

diff --git a/hugolib/page__ref.go b/hugolib/page__ref.go
new file mode 100644
index 000000000..41bd527db
--- /dev/null
+++ b/hugolib/page__ref.go
@@ -0,0 +1,117 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugolib
+
+import (
+	"fmt"
+
+	"github.com/gohugoio/hugo/common/text"
+
+	"github.com/mitchellh/mapstructure"
+	"github.com/pkg/errors"
+)
+
+func newPageRef(p *pageState) pageRef {
+	return pageRef{p: p}
+}
+
+type pageRef struct {
+	p *pageState
+}
+
+func (p pageRef) Ref(argsm map[string]interface{}) (string, error) {
+	return p.ref(argsm, p.p)
+}
+
+func (p pageRef) RefFrom(argsm map[string]interface{}, source interface{}) (string, error) {
+	return p.ref(argsm, source)
+}
+
+func (p pageRef) RelRef(argsm map[string]interface{}) (string, error) {
+	return p.relRef(argsm, p.p)
+}
+
+func (p pageRef) RelRefFrom(argsm map[string]interface{}, source interface{}) (string, error) {
+	return p.relRef(argsm, source)
+}
+
+func (p pageRef) decodeRefArgs(args map[string]interface{}) (refArgs, *Site, error) {
+	var ra refArgs
+	err := mapstructure.WeakDecode(args, &ra)
+	if err != nil {
+		return ra, nil, nil
+	}
+
+	s := p.p.s
+
+	if ra.Lang != "" && ra.Lang != p.p.s.Language().Lang {
+		// Find correct site
+		found := false
+		for _, ss := range p.p.s.h.Sites {
+			if ss.Lang() == ra.Lang {
+				found = true
+				s = ss
+			}
+		}
+
+		if !found {
+			p.p.s.siteRefLinker.logNotFound(ra.Path, fmt.Sprintf("no site found with lang %q", ra.Lang), nil, text.Position{})
+			return ra, nil, nil
+		}
+	}
+
+	return ra, s, nil
+}
+
+func (p pageRef) ref(argsm map[string]interface{}, source interface{}) (string, error) {
+	args, s, err := p.decodeRefArgs(argsm)
+	if err != nil {
+		return "", errors.Wrap(err, "invalid arguments to Ref")
+	}
+
+	if s == nil {
+		return p.p.s.siteRefLinker.notFoundURL, nil
+	}
+
+	if args.Path == "" {
+		return "", nil
+	}
+
+	return s.refLink(args.Path, source, false, args.OutputFormat)
+
+}
+
+func (p pageRef) relRef(argsm map[string]interface{}, source interface{}) (string, error) {
+	args, s, err := p.decodeRefArgs(argsm)
+	if err != nil {
+		return "", errors.Wrap(err, "invalid arguments to Ref")
+	}
+
+	if s == nil {
+		return p.p.s.siteRefLinker.notFoundURL, nil
+	}
+
+	if args.Path == "" {
+		return "", nil
+	}
+
+	return s.refLink(args.Path, source, true, args.OutputFormat)
+
+}
+
+type refArgs struct {
+	Path         string
+	Lang         string
+	OutputFormat string
+}